1. Field of the Invention
The present invention relates to a chromatograph mass spectrometer, and more specifically, to correction processing of a centroid spectrum.
2. Description of the Related Art
A mass spectrometer (MS) is often used in combination with a liquid chromatograph or a gas chromatograph (GC). A liquid chromatograph mass spectrometer (LC/MS) uses a mass spectrometer as a detector of a liquid chromatograph. The liquid chromatograph mass spectrometer introduces the mixture containing a plurality of chemical compounds to a liquid chromatograph, separates each chemical compound in a time direction by a column, introduces the component eluted from the column to a mass spectrometer via an interface portion to ionize the chemical compound, and thereafter, separates the ions on the mass number basis to detect the separated ions.
In a case of the measurement of a spectrum by an ion trap time-of-flight mass spectrometer in which the LC is combined with an ion trap mass spectrometer (IT) and a time-of-flight mass spectrometer (TOF), when ions accumulated in an ion trap are discharged to a TOF portion at a certain timing, the ions reach a detector in an increasing order of a mass number (m/z) of the ions, and are detected as signals. Thus, a period from a time when the ions are discharged from the ion trap to a time when the ions reach the detector is measured, and the intensity at which the ions reach the detector during that period is measured. As a result, the intensity of the detector signal of the ions with respect to the mass number thereof can be measured as an MS profile spectrum as shown in FIG. 3.
Regarding the spectrum information by the mass spectrometer, as shown in FIG. 5, there are a case where an MS profile spectrum (broken line portion) is displayed as it is, and a case where the mass number of the each peak in the measured MS profile spectrum is determined, and an MS centroid spectrum (vertical bar portion) converted into the representative mass number of the each peak and the intensity thereof is displayed. The mass number in a case of being converted into an MS centroid spectrum is shown at a gravity position in the peak, and the intensity is shown as an area value of the peak.
Generally, the LC/MS uses an ionization method (Electrospray “ESI”) in which an ionization procedure is soft, and atmospheric pressure chemical ionization (APCI), or the like. Therefore, unlike the case of an electron impact “EI” in the GC/MS, a simple mass-spectrum is determined, in which only ions such as [M+H]+ or [M+Na]+ with protons or a salt in a solvent added to a component are measured during positive ion measurement, and ions such as [M−H]− dehydrogenated from components are measured during negative ion measurement. Further, in a case of the ESI method, a spectrum of polyvalent ions (n≧2) such as [M+nH]n+ or [M+nNa]n+ with a plurality of protons or a salt in a solvent added to a component, depending upon the sample, is measured.
In a case of measuring a spectrum in an ionization mode in which only monovalent ions are generated as in an APCI mode, the peak of a spectrum of a component eluted from a column is detected at a position of a mass number away from a monoisotropic peak by the difference in an isotope mass number of constitutional elements of the component. Samples of a hydrocarbon type are often measured by a mass spectrometer. In a case of such samples, as shown in FIG. 4A, ions with one hydrogen atom being composed of an isotope 2H is observed at the mass number away from the monoisotopic peak by 1 m/z, and ions with two hydrogen atoms being composed of an isotope 2H are observed at the mass number away from the monoisotropic peak by 2 m/z. Thus, isotope peaks are observed at a distance of about 1 m/z. In a case where polyvalent ions are generated as in the ESI mode, the mass difference from the isotope peak varies depending upon an atomic value of the ions. As shown in FIG. 4B, in a case of divalent ions, an isotope peak is observed at a mass difference of about 0.5 m/z from an isotope peak and in a case of trivalent ions, an isotope peak is observed at a mass difference of about 0.333 m/z.
MS/MS measurement is also conducted in which the peak of a particular ion is selected from the ion peaks of a spectrum determined by the MS measurement, and the second measurement is conducted with the selected particular ion being a precursor ion. In a case of qualitative analysis carrying out a structural analysis of a component by MS/MS measurement, the mass number of a component separated from a column is often unclear. Thus, an MS/MS spectrum is measured using a procedure called Data Dependent Acquisition “DDA” in which a peak matched with a precursor ion selection condition for MS/MS measurement specified by a user is searched for from a plurality of the peaks in a spectrum at a time when a peak other than those of a medium is detected in an MS spectrum, and MS/MS spectrum measurement of the peak is carried out. As a result, information for a structural analysis by the user is provided. As also described in Patent Documents, for example, U.S. Pat. No. 6,498,340 and U.S. Pat. No. 7,009,174, the DDA is effective for the MS/MS measurement used for analyzing a compound with a complicated structure.
In the DDA, it is necessary to set measurement conditions for the user to carry out the MS/MS measurement. Examples of the typical conditions include (i) timing for starting a search for a precursor ion (a intensity threshold value of a spectrum), (ii) a search mass range of a precursor ion, and (iii) an ionic charge number of a precursor ion. When such measurement conditions are set, and a sample is injected, measurement is started. Regarding a component eluted from a column, an MS spectrum is measured by a mass spectrometer. In a case where a precursor ion matched with the measurement conditions of the DDA is searched for and found, using the MS spectrum data, the measurement of an MS/MS spectrum of a precursor ion is conducted.
When a precursor ion matched with the measurement conditions of the DDA is searched for, it is necessary that the ionic charge number of the each peak in an MS spectrum is matched with the ionic charge number specified under the selected conditions. As a procedure for calculating the ionic charge number of the each peak, various procedures have been studied. However, a procedure for carrying out charge number determination processing at a high speed, such as a search for a precursor ion mass number for conducting subsequent measurement during measurement as in a case of conducting the DDA, is limited. As one procedure, there is procedure for estimating the charge number from the difference in a mass number between adjacent peaks, using a centroid spectrum. According to this procedure, in a mass spectrometer capable of measuring the mass number precisely such as a time-of-flight mass spectrometer, an ionic charge number of a peak is estimated from the mass difference between the respective peaks in the measured/converted MS centroid spectrum.
As a peak interval becomes narrower as the charge number increases. Therefore, an overlapping effect with the adjacent peaks in a profile spectrum generating a centroid spectrum occurs. In a profile spectrum, in a case where the spectrum and the peaks before and after the spectrum are completely separated, there is no problem. However, in a case where a charge number increases, and rising or falling of the peaks before and after the spectrum is overlapped with another peak, as shown in FIG. 6A, a peak position (2) expressed by a centroid shifts from a true peak position (1). A centroid position shifts due to the overlapping of the peaks. Therefore, in a case of estimating the charge number from an interval with respect to adjacent peaks in the charge number estimation processing in the DDA, the influence of this shift becomes negligible as the charge number increases. For example, in a case where the charge number is 10, a distance with respect to the adjacent peaks is 0.100 m/z; however, in a case where the charge number is 11, the distance with respect to the adjacent peaks is 0.091 m/z. Therefore, when the peak with a charge number of 10 shifts by 0.005 m/z due to the overlapping of the peaks, the interval between the peaks becomes 0.09 m/z, so there is a possibility that the charge number may be estimated to be 11 in estimation processing. Regarding the overlapping peaks, the profile spectrum (solid line) in FIG. 6A is separated into two peak data represented by dotted lines, using a procedure called “waveform separation”, and thereafter, is converted into a centroid using information on each peak data. However, this procedure takes a time for processing since waveform separation processing is performed by differential processing (generally, tertiary differentiation) of a waveform, so this procedure cannot be conducted during the measurement processing.